The electrophotographing process has been used in various fields such as printers, facsimiles, color copying machines and high-speed copying machines. As there have been ever-increasing demands for high-quality images in these apparatuses, various developing systems have been proposed, and there are also demands for developers such as toners which commonly satisfy various features such as improved polarity control in charging, superior fluidity, etc. in accordance with various fields and required functions.
Moreover, along with the recent developments in the information and network dependent society, it is required for these apparatuses to minimize loads given to the environments. In other words, with respect to the developer, an attempt has been made to achieve long life of the developer from the viewpoint of a low-energy fixing process (low-temperature process), a reduction of toner discharge, and long life and recycling of developing vessels and developing units in printer apparatuses.
In order to satisfy these demands, toners having a modified surface, that is, so-called surface modified toners have been proposed. Examples thereof include those in which surface-modifying fine particles having various functions are anchored on the surface of a toner core particle by using a dry or wet method so as to effectively apply sufficient functions thereto, those in which core particles having a low softening temperature are coated with setting resin fine particles so as to improve the durability and fixing property, and those which are formed into a spherical shape so as to improve the chargeability and fluidity.
With respect to such a surface-modified toner, for example, Japanese Laid-Open Patent Application No. 209910/1995 (Tokukaihei 7-209910, published on Aug. 11, 1995) has disclosed a toner which is constituted at least a binder resin, a colorant and an externally adding agent, and in which to toner core particles (base particles) that have been subjected at least kneading and pulverizing processes are added and mixed the externally adding agent, and these are then subjected to a surface-modifying process in a hot air in a scattered state so that the externally adding agent is fixed onto the toner core particle.
Moreover, with respect to a manufacturing method for a surface-modified toner, U.S. Pat. No. 5,206,109 (corresponding to Japanese Laid-Open Patent Application No. 3171/1992 (Tokukaihei 4-3171) published on Jan. 8, 1992) has disclosed a method in which, after surface-modifying fine particles have been allowed to adhere to the surface of a core particle, a mechanical impact is applied thereto so that the surface-modifying fine particles are uniformly fixed on the surface of the core particle, and this is then thermally processed in a hot air flow at 200.degree. C. to 600.degree. C. so that the surface-modifying fine particles are uniformly fixed or film-formed on the surface of the core particle.
However, any of these proposals has just showed a method in which surface-modifying fine particles are fixed or film-formed on the surface of a core particle or a toner on which surface-modifying fine particles have been fixed or film-formed. Even in the case when subjected to a heating treatment under certain conditions, the added fine particles tend to be separated and isolated from the core particle due to stress that they undergo inside the developing device, if the rate of fixation of the added fine particles is low depending on processing conditions, resulting in adverse effects on image quality such as variations in the image density or fog, due to variations in the quantity of charge and the existence of isolated fine particles.
Moreover, even in the case when a sufficient fixing process has been carried out, if the toner particle is formed into a nearly complete spherical shape, the blade cleaning property to residual toner tends to deteriorate, causing an insufficient cleaning process, and subsequent degradation in the image quality.
Actually, in order to obtain a toner which allows modified fine particles on the surface of a core particle to withstand stress in practical use and has a long life without being separated and isolated from the core particle, it is essential to estimate a resulting state of toner after a certain process and to obtain a toner having desired functions based upon quantitative examinations on the state.
In the conventional technique, a specific state of the resulting toner obtained after a surface-modifying treatment is often evaluated only by visual observations on the surface of a surface-modified toner particle using an SEM (Scanning Electron Microscope), etc.; however, this fails to quantitatively confirm the state of a resulting toner and the manufacturing process, resulting in difficulties in confirming whether or not a surface-modified toner having desirable functions can be produced in a manufacturing process in question. The resulting problem is that it is highly possible that a ununiform, unstable toner is produced in each manufacturing process.
Moreover, in image-forming apparatuses using the electrophotographic system such as copying machines and printers, in general, a toner having a positive or negative charge is allowed to electrostatically adhere to an electrostatic latent image formed on a photoconductive member (photosensitive member) so as to form a toner image, and the toner image is then transferred onto a copying material such as copy paper, and then fixed thereon; thus, an image-forming process is carried out.
With respect to the above-mentioned toner, in general, toner particles having an average particle size of 5 to 20 .mu.m, which contains at least a colorant and a binder resin for fixing the colorant, etc. on a copying material such as copy paper, are used.
Conventionally, in image-forming apparatuses such as copying machines and printers using the electrophotographic system, various toners have been used as developers for developing an electrostatic latent image formed on a photosensitive member. With respect to these conventional toners, those manufactured by using a so-called pulverizing method have been known in which, to a thermoplastic resin as a binder resin are fused and kneaded a colorant, a charge control agent, an anti-offset agent (mold-releasing agent), etc., and this is cooled and solidified, and then pulverized and classified to obtain toner particles.
Moreover, other toners, obtained by a wet method have been known, and the wet method is typically exemplified by a suspension polymerization method in which a polymerizable monomer, a polymerization initiator, a colorant, etc. are mixed and dispersed together with a charge control agent, and this is polymerized in water, and a suspension granulation method in which a colorant and a charge control agent are blended with a synthetic resin and this is fused, and suspended in a non-solvent-type medium so as to be granulated,
However, those toner particles, obtained by the pulverizing method, have an irregular shape, resulting in variations in the characteristics, degradation in the fluidity and difficulty in transporting the toner through the developing device. In order to solve these problems, much attention has been focused on a method in which toner particles formed through the pulverizing method are subjected to a heating treatment and surface-modified so as to be formed into a spherical shape.
However, in the case when only a normal hot air process is applied to toner particles, problems arise in which scattering of toner particles and fog on white base occur due to elution of low fusing-point materials and an increase in the particle size caused by fusing between the toner particles. This results in inconveniences such as limited heat treatment temperatures and fusing and aggregation between toner particles in an attempt to achieve a sufficient spherical shape.
For this reason, in order to solve the above-mentioned problems, Japanese Laid-Open Patent Application No. 179363/1991 (Tokukaihei 3-179363, published on Aug. 5, 1991) has proposed a method for adding inorganic fine particles to the surface of a toner particle, etc. Moreover, Japanese Laid-Open Patent Application No. 179363/1991 has also disclosed a toner obtained from a method in which, after a toner containing at least one kind selected from the group consisting of wax, a higher fatty acid, polyolefin and an olefin copolymer has been formed by a pulverizing method as a surface-modified toner, the toner is formed into a spherical shape through a heating treatment, and in this method, inorganic fine particles are preliminarily allowed to adhere to the toner surface.
However, any of these proposals has just showed a method in which surface-modifying fine particles are fixed or film-formed on the surface of a core particle (base particle) or a toner on which surface-modifying fine particles have been fixed or film-formed, and has not define anything about the amount of inorganic fine particles to be added.
However, in the case when the addition of inorganic fine particles exceeds the amount that is actually required, that is, in the case when the added inorganic fine particles not only sufficiently cover the surface of the core particle, but also cause many isolated inorganic fine particles, there might be serious degradation in fixing property to the surface of paper and a reduction in the effect for preventing offset, as well as variations in the quantity of charge and image density, and degradation in the image quality such as filming, toner scattering and fog.
In contrast, in the case when the amount of addition of the inorganic fine particles is small and the inorganic fine particles fail to sufficiently cover the surface of the core particle, since the fixation and film formation of the inorganic fine particles are insufficient, the durability, which is a major effect of the heating treatment, is not improved, and fine particles separated from the toner particles due to stress imposed thereon inside the developing device might cause fusing onto the blade, etc.
In other words, in order to obtain toner particles which are superior in fixing property and anti-offset property, and resistant to stress in practical use with life property free from separation etc., it is essential to properly define the amount of addition of the inorganic fine particles, and to regulate the amount of addition of the inorganic fine particles by quantitatively confirming the relationship between the amount of addition of the inorganic fine particles and the fixing property and the durability as well as influences of the amount of addition of the inorganic fine particles to the toner characteristics. However, there have not been any examples which quantitatively confirm the relationship between the amount of addition of the inorganic fine particles and these factors.